Water filtration systems are used to purify water. For example, a reverse osmosis (RO) system includes an RO membrane. When feed water enters the system, some water goes through the membrane as purified permeate and some is left as concentrate. A shut-off valve can be used to control water flow through the system. For example, the shut-off valve can be placed between the feed line and the permeate line. When the pressure drops in the permeate line, the shut-off valve opens the feed line and water flows though the system. When the pressure rises in the permeate line, the shut-off valve closes and water stops flowing. However, sometimes the shut-off valve does not close all the way and feed water may continue to run to drain.